A cylinder head is a main part of an engine, providing space for the passages that feed air and fuel to the cylinder and that allow exhaust to escape. Usually, the explosion within a combustion chamber heats the bottom of a cylinder head to up to approximately 200° C. With the increase in the temperature of the combustion chamber, the fuel is likely to spontaneously ignite, inducing the occurrence of a knocking phenomenon. Engine knocking, if occurring, degrades the durability of the engine and lowers fuel efficiency.
To prevent a knocking phenomenon in the combustion chamber, the heat generated after the explosion should be rapidly released. Therefore, a cylinder head made of a material having high thermal conductivity is capable of rapidly releasing the heat transferred from the combustion chamber to the head, which leads to an increase in fuel efficiency.
Conventional cylinder heads for gasoline engines are constructed by molding an Al—Si—Cu-based alloy, AC2B, through gravity casting, and then by T7 heat treatment.
The AC2B alloy includes, by weight, Si: 5.5˜6.5%, Fe: 1.0%, Cu: 3.0˜4.0%, Mn: 0.6%, Mg: 0.1%, Ni: 0.35%, Zn: 1.0%, a balance of Al, and inevitable impurities to form 100%.
As for the physical properties of the AC2B alloy having this composition, the AC2B alloy, after T7 heat treatment, exhibits a yield strength of 220 MPa or higher, a tensile strength of 270 MPa or higher, an elongation rate of 1.0% or higher, and a thermal conductivity of 160 W/mK@25° C. and 165 W/mK@ 200° C.
An AC2B alloy can increase in strength and castability with a higher content of an Al2Cu strengthener and a Si precipitate. However, the formation of excessive precipitates may be a cause of reduced thermal conductivity.
A cylinder head is required to maintain high strength and thermal conductivity at high temperatures. Although having sufficient strength, conventional AC2B alloys are deficient in thermal conductivity.
There is, therefore, a need for a novel aluminum alloy that allows the cylinder head to maintain excellent thermal conductivity at the high temperature (200° C.) that occurs during the operation of the cylinder while having strength as high as or higher than conventional alloys.